1. Field of the Invention
In general, the present invention relates to a signal receiving apparatus, a signal receiving method adopted by the signal receiving apparatus and a program implementing the signal receiving method. In particular, the embodiments of the present invention relates to a signal receiving apparatus capable of easily carrying out an OFDM (Orthogonal Frequency Division Multiplexing) demodulation process making use of a window function without causing an ISI (Inter Symbol Interference) even in a multipath environment, relates to a signal receiving method adopted by the signal receiving apparatus and relates to a program implementing the signal receiving method.
2. Description of the Related Art
As a method for modulating a ground digital broadcast, there has been proposed an OFDM (Orthogonal Frequency Division Multiplexing) method for modulating a number of various orthogonal carrier waves by adoption a PSK (Phase Shift Keying) technique and/or a QAM (Quadrature Amplitude Modulation) technique (Japanese Patent Laid-Open No. 2005-303440).
The OFDM method is defined in that, since the entire transmission band is divided into a number of transmission sub-bands each allocated to a sub-carrier wave, the width of the transmission sub-band is small, giving rise to a low transmission speed even though the total transmission speed does not change from the speed according to the existing modulation method.
In addition, the OFDM method is also defined in that, since a number of sub-carrier waves are transmitted, the symbol speed is low. Thus, the multipath time length relative to the time length of one symbol can be reduced. As a result, the OFDM method also has a characteristic that a transmission according to the OFDM method is hardly affected by the multipath.
On top of that, the OFDM method is also defined in that, since data is assigned to a plurality of sub-carrier waves, a signal transmitting circuit can be constructed by making use of an IFFT (Inverse Fast Fourier Transform) processing circuit for carrying out an inverse Fourier transform operation at a modulation time whereas a signal receiving circuit can be constructed by making use of an FFT (Fast Fourier Transform) processing circuit for carrying out a Fourier transform operation at a demodulation time.
FIG. 1 is a diagram showing a sequence of OFDM symbols.
In accordance with the OFDM method, a signal is transmitted in units each referred to as an OFDM symbol.
As shown in the diagram of FIG. 1, one OFDM symbol includes an effective symbol and a guard interval referred to hereafter simply as a GI. The effective symbol is a signal segment on which an IFFT operation is carried out at a transmission time. On the other hand, the GI is a copy of the waveform of a later portion of the effective symbol. The GI is inserted into a position existing on the time axis as a position in the front of the effective symbol.
By inserting such a GI in accordance with the OFDM method, the OFDM method is capable of preventing an ISI (Inter Symbol Interference, which is an interference between symbols) from occurring in a multipath environment.
A plurality of such OFDM symbols are collected to form an OFDM-transmitted frame. For example, in accordance with ISDB-T (Integrated Services Digital Broadcasting-Terrestrial) specifications, 204 OFDM symbols form an OFDM-transmitted frame. The ISDB-T specifications are specifications prescribing ground digital broadcasts transmitted in Japan.
In accordance with the OFDM method, by using an OSDM-transmitted frame unit as a reference, each of a data carrier wave, an SP (Spread Pilot) signal and a TMCC carrier wave is used as a sub-carrier wave as shown in a diagram of FIG. 2. The data carrier wave is a carrier wave for transmitting data. The SP signal is a signal used for inferring a transmission characteristic (or a frequency characteristic) on the OFDM receiver side. The TMCC carrier wave is a carrier wave for transmitting transmission parameters.
Placed at fixed intervals, the SP signals are each a complex vector having a known amplitude and a known phase. The data sub-carrier wave conveying data to be transmitted is placed between SP signals. An OFDM receiver receives an SP signal in a state distorted due to an effect of the characteristic of the transmission line. Since the state of the SP signal at a signal transmitting time is known, however, the characteristic of the transmission line can be inferred by comparing the state of the SP signal at a signal transmitting time with the state of the SP signal at a signal receiving time.
FIG. 3 is a block diagram showing a typical configuration of the existing OFDM receiver.
As shown in the block diagram of FIG. 3, the existing OFDM receiver employs an antenna 1, a frequency conversion circuit 2, a local oscillation circuit 3, an A/D (Analog/Digital) conversion circuit 4, a orthogonal-demodulation circuit 5, a local oscillation circuit 6, an FFT circuit 7, an SP-signal extraction circuit 8, a time-direction transmission-line characteristic inference circuit 9, a frequency-direction transmission-line characteristic inference circuit 10, a transmission-line compensation circuit 11, a carrier-frequency synchronization circuit 12, a window reproduction circuit 13 and a delay-profile inference circuit 14.
The frequency conversion circuit 2 is a section for multiplying an RF signal received from the antenna 1 by a carrier wave received from the local oscillation circuit 3 as a carrier wave with an oscillation frequency of (fc+fIF) in order to convert the RF signal into an IF signal having a center frequency fIF and for outputting the IF signal obtained as a result of the frequency conversion process to the A/D conversion circuit 4.
The A/D conversion circuit 4 is a section for carrying out an A/D conversion process of converting the analog IF signal received from the frequency conversion circuit 2 into a digital IF signal and outputting the digital IF signal obtained as a result of the A/D conversion process to the orthogonal-demodulation circuit 5.
The orthogonal-demodulation circuit 5 is a section for carrying out an orthogonal demodulation process making use of a carrier wave received from the local oscillation circuit 6 as a carrier wave having the frequency fIF in order to acquire a baseband OFDM signal from the digital IF signal supplied by the A/D conversion circuit 4. The baseband OFDM signal is the so-called time-domain OFDM signal not subjected yet to an FFT process. The orthogonal-demodulation circuit 5 outputs the time-domain OFDM signal obtained as a result of the orthogonal demodulation process to the carrier-frequency synchronization circuit 12, the FFT circuit 7 and the window reproduction circuit 13.
The FFT circuit 7 is a section for removing a GI from the time-domain OFDM signal on the basis of an FFT window position specified by the window reproduction circuit 13 in order to extract a signal in a range including an effective symbol associated included in the same OFDM symbol as the removed GI from the time-domain OFDM signal.
The FFT circuit 7 is also a section for carrying out an FFT process on the extracted signal as a demodulation process in order to generate a post-demodulation OFDM signal and outputting the generated post-demodulation OFDM signal to the SP-signal extraction circuit 8 and the transmission-line compensation circuit 11. The post-demodulation OFDM signal generated by the FFT circuit 7 is the so-called frequency-domain OFDM signal which is obtained as a result of the FFT process.
The start position of the FFT process carried out by the FFT circuit 7 is a position between positions A and B shown in the diagram of FIG. 1. The position A is a position on a boundary between two adjacent OFDM symbols whereas the position B is a position on a boundary between an effective symbol included in the later one of the two adjacent OFDM symbols and a GI included in the same later OFDM symbol as the effective symbol. The range of the FFT process is referred to as an FFT window. Referred to hereafter as an FFT window position cited above, the start position of the FFT window is specified by the window reproduction circuit 13.
The SP-signal extraction circuit 8 is a section for extracting an SP signal from the frequency-domain OFDM signal received from the FFT circuit 7 and removing modulation components from the extracted SP signal in order to infer a transmission-line characteristic prevailing at the position of an OFDM symbol associated with the SP signal as a transmission-line characteristic of a sub-carrier wave. The SP-signal extraction circuit 8 outputs a signal representing the transmission-line characteristic of the sub-carrier wave to the time-direction transmission-line characteristic inference circuit 9.
The time-direction transmission-line characteristic inference circuit 9 is a section for inferring transmission-line characteristics prevailing at positions of other OFDM symbols lined up in the time-axis direction (which is also referred to as an OFDM-symbol direction) in an area between the positions of specific OFDM symbols each associated with an SP signal as transmission-line characteristics of a sub-carrier wave on the basis of the transmission-line characteristics inferred by the SP-signal extraction circuit 8 as the transmission-line characteristics which prevail at the positions of the specific OFDM symbols as the transmission-line characteristics of the sub-carrier wave. In the diagram of FIG. 2, the vertical direction is the time-axis direction whereas the horizontal direction is a frequency direction.
Let us assume for example that the SP-signal extraction circuit 8 infers transmission-line characteristics prevailing at the positions of specific OFDM symbols associated with SP signals SP1 and SP2 shown in the diagram of FIG. 2 as transmission-line characteristics of a sub-carrier wave having a sub-carrier number of 0. In this case, the time-direction transmission-line characteristic inference circuit 9 infers transmission-line characteristics prevailing at the positions of other OFDM symbols in an area A1 sandwiched between the positions of the specific OFDM symbols as shown in the diagram of FIG. 2 as transmission-line characteristics of the sub-carrier wave by making use of transmission-line characteristics inferred by the SP-signal extraction circuit 8.
As shown in the diagram of FIG. 2, since a plurality of SP signals are inserted for every 3 sub-carrier waves, the time-direction transmission-line characteristic inference circuit 9 infers transmission-line characteristics prevailing at positions of other OFDM symbols lined up in the time-axis direction (which is also referred to as an OFDM-symbol direction) in an area between the positions of specific OFDM symbols each associated with one of the inserted SP signals as transmission-line characteristics of a sub-carrier wave. For every 3 sub-carrier waves, the time-direction transmission-line characteristic inference circuit 9 outputs a signal representing transmission-line characteristics inferred thereby as the transmission-line characteristics of the three sub-carrier waves to the frequency-direction transmission-line characteristic inference circuit 10 and the delay-profile inference circuit 14. The transmission-line characteristics represented by the signal output by the time-direction transmission-line characteristic inference circuit 9 include the transmission-line characteristics inferred by the SP-signal extraction circuit 8.
The frequency-direction transmission-line characteristic inference circuit 10 is a section for inferring transmission-line characteristics prevailing at positions of OFDM symbols lined up in the frequency direction (which is also referred to as a sub-carrier direction) as transmission-line characteristics of sub-carrier waves on the basis of transmission-line characteristics output by the time-direction transmission-line characteristic inference circuit 9.
For example, the frequency-direction transmission-line characteristic inference circuit 10 infers transmission-line characteristics prevailing at positions of an OFDM symbol included in an area A2 shown in the diagram of FIG. 2 as OFDM symbol #3 conveyed at the same time by sub-carrier waves having different sub-carrier numbers as shown in the diagram of FIG. 2 as transmission-line characteristics of the sub-carrier waves. However, the frequency-direction transmission-line characteristic inference circuit 10 infers merely transmission-line characteristics which have not been inferred yet by the SP-signal extraction circuit 8 and the time-direction transmission-line characteristic inference circuit 9. Thus, the frequency-direction transmission-line characteristic inference circuit 10 infers transmission-line characteristics by making use of transmission-line characteristics inferred by the SP-signal extraction circuit 8 and the time-direction transmission-line characteristic inference circuit 9.
As a result, the transmission-line characteristics prevailing at the positions of all OFDM symbols as transmission-line characteristics of sub-carrier waves are inferred. The frequency-direction transmission-line characteristic inference circuit 10 outputs a signal representing the inferred transmission-line characteristics to the transmission-line compensation circuit 11.
The transmission-line compensation circuit 11 is a section for removing distortion components, which are included in the frequency-domain OFDM signal received from the FFT circuit 7 as components attributed to distortions occurring along the transmission line, by making use of transmission-line characteristics represented by a signal output by the frequency-direction transmission-line characteristic inference circuit 10. The transmission-line compensation circuit 11 outputs a frequency-domain OFDM signal including no distortion components to circuits at a stage succeeding this OFDM receiver as an equalizer output signal.
The carrier-frequency synchronization circuit 12 is a section for controlling the local oscillation circuit 6 on the basis of the time-domain OFDM signal received from the orthogonal-demodulation circuit 5 in order to drive the local oscillation circuit 6 to output a carrier wave having an oscillation frequency fIF synchronized with the frequency of the IF signal generated by the frequency conversion circuit 2 to the orthogonal-demodulation circuit 5.
The window reproduction circuit 13 is a section for determining an FFT window position on the basis of the time-domain OFDM signal received from the orthogonal-demodulation circuit 5 and a delay-profile signal received from the delay-profile inference circuit 14 and outputting the determined FFT window position to the FFT circuit 7.
The delay-profile inference circuit 14 is a section for inferring a delay profile of the transmission line by finding a time response characteristic of the transmission line and outputting a signal representing the inferred delay profile to the window reproduction circuit 13.
Typically, the delay-profile inference circuit 14 carries out an IFFT process on transmission-line characteristics represented by a signal output by the time-direction transmission-line characteristic inference circuit 9 in order to infer the delay profile of the transmission line. Since the transmission-line characteristic represented by a signal output by the time-direction transmission-line characteristic inference circuit 9 is a frequency characteristic, a time response characteristic obtained as a result of the IFFT process carried out on the transmission-line characteristic is a delay profile.
FIG. 4 is a block diagram showing a typical configuration of the FFT circuit 7 employed in the existing OFDM receiver shown in the block diagram of FIG. 3.
As shown in the block diagram of FIG. 4, the FFT circuit 7 employs a GI removal circuit 7-1 and an FFT processing circuit 7-2.
The GI removal circuit 7-1 is a section for removing a GI included in a time-domain OFDM signal received from the orthogonal-demodulation circuit 5 on the basis of an FFT window position received from the window reproduction circuit 13. The GI removal circuit 7-1 outputs an effective symbol obtained as a result of the process to remove a GI included in a time-domain OFDM signal to the FFT processing circuit 7-2.
The FFT processing circuit 7-2 is a section for carrying an FFT process on an effective symbol received from the GI removal circuit 7-1 in order to transform a time-domain OFDM signal including the effective symbol into a frequency-domain OFDM signal. The FFT processing circuit 7-2 outputs the frequency-domain OFDM signal obtained as a result of the FFT process to the SP-signal extraction circuit 8 and the transmission-line compensation circuit 11.
The FFT window position which is the start of the FFT process mentioned above is explained by referring to diagrams of FIGS. 5 and 6 as follows.
FIG. 5 is a diagram showing a typical FFT window position in a 1-wave environment whereas FIG. 6 is a diagram showing a typical FFT window position in a multipath environment.
In the 1-wave environment, an OFDM signal consisting of merely a principal wave as shown in the diagram of FIG. 5 is received by the OFDM receiver. In this diagram, notation Tu denotes the length of an effective wave represented by the OFDM signal whereas notation Tg denotes the length of a GI included in the same OFDM signal as the effective wave.
The FFT window position is a position between positions C and D shown in the diagram of FIG. 5. The position C is a position on a boundary between two adjacent OFDM symbols whereas the position D is a position on a boundary between an effective symbol of the later one of the two adjacent OFDM symbols and a GI included in the same later OFDM symbol as the effective symbol. In the case of the typical 1-wave environment shown in the diagram of FIG. 5, the FFT window position is specified to be the position D. As described above, the GI removal circuit 7-1 removes a GI included in a time-domain OFDM signal received from the orthogonal-demodulation circuit 5 on the basis of an FFT window position received from the window reproduction circuit 13. A range starting from the FFT window position as a range consisting of N FFT points is used as an FFT window. Notation N also denotes the number of samples in the effective symbol.
In the multipath environment, on the other hand, an OFDM signal consisting of the principal wave and a delayed wave as shown in the diagram of FIG. 6 is received by the OFDM receiver. For the sake of the convenience of the explanation, a multipath environment of two waves, i.e., the principal wave and a delayed wave, is explained.
In the case of the typical multipath environment shown in the diagram of FIG. 6, the OFDM receiver receives a compound wave consisting of the principal wave, which serves as a direct wave, and a delayed wave. The delayed wave is a wave which is delayed from the principal wave by a delay time τ and has an attenuated amplitude. As shown in the diagram of FIG. 6, the width of a band representing the principal wave is made greater than the width of a band representing the delayed wave in order to indicate that the amplitude of the principal wave is greater than the amplitude of the delayed wave.
In the case of the typical multipath environment shown in the diagram of FIG. 6, in order to prevent an ISI (Inter-Symbol Interference) from occurring after a demodulation process, the FFT window position is set at a position between positions E and F shown in the diagram of FIG. 6. The position E is the start position of an OFDM symbol represented in the delayed wave. There is no interference in this OFDM symbol from an OFDM symbol immediately preceding this OFDM symbol. On the other hand, the position F is a position on a boundary between an effective symbol represented by the principal wave and a GI included in the same OFDM symbol as the effective symbol in the principal wave. Thus, in the case of a typical multipath environment, the range in which the FFT window position can be specified is narrow in comparison with the range in a 1-wave environment.
In the case of the typical multipath environment shown in the diagram of FIG. 6, the FFT window position is specified to be the position F which is a position on a boundary between an effective symbol represented by the principal wave and a GI included in the same OFDM symbol as the effective symbol in the principal wave.
An OFDM demodulation process making use of an FFT window is carried out as follows.
Muschallik C., ‘Improving an OFDM reception using an adaptive Nyquist windowing’, Consumer Electronics, IEEE Transactions, Volume 42, Issue 3, August 1996, Pages 259-269 discloses an OFDM demodulation process making use of a window function.
FIG. 7 is a block diagram showing a typical configuration of an FFT circuit 7 for carrying out an OFDM demodulation process making use of a window function.
As shown in the block diagram of FIG. 7, the FFT circuit 7 employs a window-function utilization circuit 7-11, a 0 addition circuit 7-12, a 2N-point FFT processing circuit 7-13 and an even-numbered carrier extraction circuit 7-14. In the same way of the FFT circuit 7 shown in the block diagram of FIG. 4, the FFT circuit 7 having a configuration like the one shown in the block diagram of FIG. 7 receives a time-domain OFDM signal from the orthogonal-demodulation circuit 5.
The window-function utilization circuit 7-11 is a section for setting a window function for a time-domain OFDM signal received from the orthogonal-demodulation circuit 5 on the basis of an FFT window position specified by the window reproduction circuit 13 and multiplying the time-domain OFDM signal by the window function in order to produce a weighted time-domain OFDM signal. The window-function utilization circuit 7-11 outputs the weighted time-domain OFDM signal obtained as a result of the operation to multiply the time-domain OFDM signal received from the orthogonal-demodulation circuit 5 by the window function to the 0 addition circuit 7-12. In the following description, the weighted time-domain OFDM signal is also referred to as a post-multiplication time-domain OFDM signal.
In order to allow an FFT process to be carried out on a time-domain OFDM signal having a format also including a GI, as shown in a diagram of FIG. 10C, the 0 addition circuit 7-12 adds 0 values, the number of which is determined in advance, to the tail of the weighted time-domain OFDM signal received from the window-function utilization circuit 7-11 and converts the weighted time-domain OFDM signal having 0s added thereto into a signal including 2N samples where notation N denotes the number of samples in an effective symbol. The 0 addition circuit 7-12 outputs the time-domain OFDM signal including the 2N samples to the 2N-point FFT processing circuit 7-13.
The 2N-point FFT processing circuit 7-13 is a section for carrying out a 2N-point FFT process on the 2N samples included in the time-domain OFDM signal having a GI in order to transform the time-domain OFDM signal including 2N samples into a frequency-domain OFDM signal. The 2N-point FFT processing circuit 7-13 outputs the frequency-domain OFDM signal obtained as a result of the FFT process to the even-numbered carrier extraction circuit 7-14.
The even-numbered carrier extraction circuit 7-14 is a section for extracting the signal of an even-numbered carrier wave from the frequency-domain OFDM signal supplied by the 2N-point FFT processing circuit 7-13 and outputting the extracted signal as a frequency-domain OFDM signal to the SP-signal extraction circuit 8 and the transmission-line compensation circuit 11.
FIG. 8 is a plurality of diagrams showing a time-domain OFDM signal and typical window functions.
To be more specific, FIG. 8A is a diagram showing a time-domain OFDM signal representing one OFDM symbol. FIG. 8B is a diagram showing a trapezoidal window function whereas FIG. 8C is a diagram showing a protruding window function. FIG. 8D is a diagram showing a raised cosine window function. In the diagrams of FIG. 8, the horizontal axis represents the lapse of time whereas the vertical axis represents the amplitude of time-domain OFDM signal and the values of the window functions. Each of the values of the window functions is used as a multiplier by which the amplitude of the time-domain OFDM signal is to be multiplied.
A time segment S1 in the diagrams of FIG. 8 is a time segment corresponding to the time segment of the GI whereas a time segment S2 is a time segment starting from the boundary between the GI and the effective symbol and ending at a position leading ahead of the end of the effective symbol by the length Tg of the GI. A time segment S3 is a time segment between the position leading ahead of the end of the effective symbol by the length Tg of the GI and the end of the effective symbol.
As shown in the diagrams of FIG. 8, each of the window functions has a fixed window width of (Tu+Tg) and a predetermined value varying in the range 0 to 1 as a function of time.
With the start position of each window function coinciding with the start position of an OFDM symbol, the trapezoidal window function shown in the diagram of FIG. 8B has a value rising from 0 to 1 along a straight line over the time segment S1 and the fixed value of 1 throughout the time segment S2. The trapezoidal window function has a value decreasing from 1 to 0 along a straight line over the time segment S3.
The protruding window function shown in the diagram of FIG. 8C has a fixed value of 0.5 throughout the time segment S1 and a fixed value of 1 throughout the time segment S2. The protruding window function has the fixed value of 0.5 throughout the time segment S3.
The raised cosine window function shown in the diagram of FIG. 8D has a value rising from 0 to 1 along a curved line over the time segment S1 and the fixed value of 1 throughout the time segment S2. The raised cosine window function has a value decreasing from 1 to 0 along a curved line over the time segment S3.
A window function is defined in that, if the time segment S3 of the window function itself overlaps the time segment S1 of the window function shifted by a distance corresponding to the length Tu of the effective symbol in the time lapsing direction along the time axis as shown in a diagram of FIG. 9, the sum of the values of the window function itself and the shifted window function in the overlapping time segments S1 and S3 each having the GI length Tg is equal to 1 throughout the overlapping time segments S1 and S3.
For example, a trapezoidal window function W′ shown in the diagram of FIG. 9 is a window function obtained as a result of shifting a trapezoidal window function W by a distance corresponding to the length Tu of the effective symbol in the time lapsing direction along the time axis. Times t1 and t2 are any arbitrary points of time in the overlapping time segment S3 of the trapezoidal window function W and the overlapping time segment S1 of the trapezoidal window function W′. At each of the times t1 and t2, the sum of the values of the trapezoidal window functions W and W′ is equal to 1.
FIG. 10 is a plurality of explanatory diagrams each referred to in description of an OFDM demodulation process carried out by making use of the trapezoidal window function shown in the diagram of FIG. 8.
To be more specific, FIG. 10A is a diagram showing a time-domain OFDM signal representing one OFDM symbol whereas FIG. 10B is a diagram showing the trapezoidal window function. FIG. 10C is a diagram showing a time-domain OFDM signal supplied to the 2N-point FFT processing circuit 7-13 employed in the FFT circuit shown in the block diagram of FIG. 7.
The window-function utilization circuit 7-11 sets the trapezoidal window function shown in the diagram of FIG. 10B. As shown in this diagram, a position G is the start position of the time segment S2 of the trapezoidal window function. As described earlier, the trapezoidal window function has a fixed value of 1 throughout the time segment S2. The window-function utilization circuit 7-11 sets a trapezoidal window function that has a start position G coinciding with an FFT window position, which is specified by the window reproduction circuit 13, on the time axis. Then, the window-function utilization circuit 7-11 multiplies the time-domain OFDM signal shown in the diagram of FIG. 10A by the trapezoidal window function shown in the diagram of FIG. 10B in order to produce a weighted time-domain OFDM signal shown in the diagram of FIG. 10C.
In an OFDM receiver carrying out an OFDM demodulation process by making use of a window function, the position on the boundary between the GI and the effective symbol is an optimum FFT window position. In the typical OFDM demodulation process explained by referring to the diagrams of FIG. 10, the window reproduction circuit 13 specifies the optimum position as an FFT window position.
As described above, in this typical OFDM demodulation process, the window-function utilization circuit 7-11 multiplies the time-domain OFDM signal shown in the diagram of FIG. 10A by the trapezoidal window function shown in the diagram of FIG. 10B in order to produce a weighted time-domain OFDM signal having the same trapezoidal shape as the trapezoidal window function as shown in the diagram of FIG. 10C and supplies the weighted time-domain OFDM signal to the 0 addition circuit 7-12.
The 0 addition circuit 7-12 adds 0 values, the number of which is determined in advance, to the tail of the weighted time-domain OFDM signal received from the window-function utilization circuit 7-11 and converts the weighted time-domain OFDM signal into a signal including 2N samples as shown in the diagram of FIG. 10C where notation N denotes the number of samples in the effective symbol. The 0 addition circuit 7-12 then outputs the time-domain OFDM signal including the 2N samples to the 2N-point FFT processing circuit 7-13. The 2N-point FFT processing circuit 7-13 carries out a 2N-point FFT process on the 2N samples included in the time-domain OFDM signal having a GI in order to transform the time-domain OFDM signal including the 2N samples into a frequency-domain OFDM signal.
By making use of a GI, which is a copy of a later portion of an effective symbol included in the same OFDM symbol as the GI, in a demodulation process as described above, it is possible to provide the OFDM receiver with a characteristic of resistance against noises.